This invention relates in general to evaporation and distillation apparatus of the orbital or "wobbling" type. More specifically, it relates to an improved evaporator with systems to decrease thermal resistance at the condensing and evaporation surfaces of a heat transfer tube and to control thermal resistance due to fouling at the evaporation surface
Evaporation and distillation apparatus of the orbital tube type are described in U.S. Pat. Nos. 4,230,529; 4,441,963; 4,618,399; and 4,762,592 to one of the present applicants. The evaporators in these patents have a container that holds one or more vertically oriented heat transfer tubes. The tubes are smooth surfaced, circular in cross section, open at both ends, and made of a material with good heat conductivity properties. A distributor directs a feed liquid to the interior of each tube. The orbital motion spreads the liquid into a film. Heat transferred radially through the wall of the tube evaporates a portion of the feed liquid into a vapor stream. In one form of evaporator, a compressor draws the evaporated vapor from a vapor chamber, compresses it, and then discharges the vapor to a condensation chamber. The compressor and partitions isolate the condensation chamber from the vapor chamber open to the interior of the tubes. The compression heats the vapor, resulting in a condensing temperature outside the tubes higher than the boiling temperature inside the tubes. Condensation of the vapor releases heat which is used to produce the evaporation. The container has an outlet for the condensate which collects near the bottom of the container.
The '963 patent, in part, describes a rigid bar that is positively driven to rotate within the tube to spread viscous liquids into a thinner, more evenly distributed film than would be possible relying solely on the orbital motion to distribute the liquid. The '399 patent describes a whip rod located in the tube which spreads the feed liquid, of any viscosity, into a highly thin and uniform film to reduce its thermal resistance and to enhance its evaporation. The whip rod also removes scales and other solid residue of evaporation that can foul the interior surface and impede the heat transfer through the tube. The '399 patent discloses several arrangements for mounting the rod, including lengths of cables, a flexible, but non rotating anchor connected between a base and the lower end of the rod, and a double universal joint also connected between the lower end of the whip rod and the base. While the whip rod has proven to be quite effective as a film distributor, the mounting arrangements have disadvantages. They increase the overall material, assembly and operating costs. Also, they fail. Material fatigue of flexible cables supporting the whip rods is a particular concern.
More generally, the combination of an orbiting vertical heat transfer tube and a whip rod is intended to facilitate the transfer of the heat of condensation at the exterior surface of the tube to the interior surface where it drives the evaporation. The effectiveness of this heat transfer through the tube is expressed as the total heat transfer coefficient U, which in turn can be expressed as the reciprocal of the thermal resistances to this flow as follows: ##EQU1## where R.sub.e =thermal resistance of the evaporation side of the tube wall
R.sub.w =thermal resistance of the wall PA1 R.sub.c =thermal resistance of the condensing side of the tube wall PA1 R.sub.f =the thermal resistance due to the accumulation of solids deposited upon the tube surface after extended use.
The known whip rod reduces R.sub.e and R.sub.f. However, an important and often limiting factor on the ultimate effectiveness of such orbital evaporators is R.sub.c. This thermal resistance is due principally to the accumulation of condensate and to any non condensible gases carried by the vapor to the exterior wall of the tube.
Heretofore flow of the condensate has depended on the force of the orbiting motion and on gravity. In practice a substantial thickness of condensate sheet can develop, particularly at the lower end of the tube. Heretofore in orbital tube evaporators there has been no arrangement to reduce the R.sub.c of the accumulated condensate layer.
It is therefore a principal object of this invention to improve the efficiency of orbital tube evaporation/distillation apparatus by providing a reduction in the thermal resistance at the condensing surface of the tube.
Another principal object is to provide a system for controlling the thermal resistance at the evaporation surface of the tube due to the presence of the feed liquid and fouling.
Another principal object is to provide the foregoing advantages at a low cost of manufacture and in a manner that is compatible with the operation of known evaporators of the orbital tube type.
A further object is to provide an improved whip rod and whip rod mounting arrangement that avoids flexible cables, anchors and universal joints to decrease the cost and to enhance the reliability of the evaporator.
Still another object is to provide an improved evaporator with all of the foregoing advantages which is also compact.
A still further object of this invention is to provide an orbital tube evaporator and method of operation which controls fouling to a degree that the evaporator can be operated for an indefinite period of time, that is, without periodic shut downs to clean fouled tubes, particularly in connection with the desalinization of seawater.